1. Field of the Invention
The present invention relates to an imaging apparatus and a lens movement control method.
2. Description of the Related Art
Recently, as typified by cellular phones, mobile devices with a built-in camera as an imaging apparatus are rapidly becoming popular. There is an increasing number of cameras each to be built in a mobile device, which have an auto focus mechanism. Because miniaturization is a prerequisite in mobile devices, the miniaturization of the auto focus mechanism is desired.
Conventionally, a stepping motor as a lens drive motor is used to do auto focusing. An auto focus mechanism using the stepping motor needs to convert the torque of the motor into the movement of a lens, making miniaturization difficult.
There are methods which do not use an electromagnetic motor like a stepping motor. One example of such methods drives a lens by an ultrasonic motor used by, for example, a single-lens reflex camera, as disclosed in Unexamined Japanese Patent Application KOKAI publication No. 2005-57839.
However, because the ultrasonic motor employs a mechanism which generates ultrasonic vibration by a plurality of piezoelectric elements, miniaturization and cost reduction are difficult.
As a lens moving mechanism which overcomes the foregoing problem, there is a Smooth Impact Drive Mechanism (hereinafter, “SIDM”) as disclosed in Unexamined Japanese Patent Application KOKAI publication No. 2002-95272 and Unexamined Japanese Patent Application KOKAI publication No. 2005-86887.
In the SIDM, as shown in FIG. 15, one end and the other end of a piezoelectric element 1 are respectively attached to a fixation member 2 and a drive shaft 3. A Flexible Print Circuit (hereinafter, “FPC”) 4 is attached to the piezoelectric element 1, and a voltage is applied to the piezoelectric element 1 through the FPC 4. A lens holder 5 for fixing a lens is mounted on the drive shaft 3 or attached thereto in such a way that the drive shaft 3 penetrates the lens holder 5. The lens holder 5 moves along the drive shaft 3. The position of the lens is determined based on at what position in the drive shaft 3 the lens holder 5 is located. A best focal length which is a distance for best focusing changes in accordance with the position of the lens.
It is generally known that the piezoelectric element 1 expands and contracts as a voltage is applied. As a pulse voltage is repeatedly applied to the piezoelectric element 1 via the FPC 4, the piezoelectric element 1 expands and contracts based on the pulse voltage. Adjusting the rise speed of the pulse and the fall speed thereof can adjust the expansion and contraction speed of the piezoelectric element 1.
As the piezoelectric element 1 is slowly expanded, the lens holder 5 is displaced with the displacement of the piezoelectric element 1 because of friction between the lens holder 5 and the drive shaft 3. As the piezoelectric element 1 is rapidly contracted, the frictional part of the lens holder 5 slips because of the inertia, and the lens holder 5 remains at almost the same position. By repeating expansion and contraction, the lens is displaced by a long stroke.
As the foregoing operation is performed reversely, the lens can be moved in the opposite direction.
In the case of auto focusing, the camera moves the lens to plural points within a desired focus range by controlling the lens moving mechanism, finds out the best focus lens position where the best focus is set to an imaging target from image information obtained at each point, and moves the lens to that position. The method of finding the best focus lens position is disclosed in Unexamined Japanese Patent Application KOKAI publication No. H5-122579.
When the lens is moved to set the best focus to the imaging target, a lens position (reference position) to be a reference where a focal position is specified beforehand is necessary to figure out a current lens position. Thus, when the lens is mounted on the camera, a stopper which stops the lens holder 5 at a position over the drive shaft 3 where the best focal length becomes infinite (hereinafter, this position is called “infinity end”) is set up, and this infinity end is taken as a reference position. The reference position may not be the infinity end, but may be set to a position where the best focal length is 1.4 m or a position where the best focal length is 1 m, both positions being shifted from the infinity end by specified number of pulses.
In the SIDM, pulse control is performed, and the movement of the lens holder 5 is controlled by the number of pulses or a time when pulses are applied. In moving the lens holder 5 to a desired position, first, the lens holder 5 is moved until it hits the stopper which serves as the reference position. From this position, a pulse voltage having a necessary number of pulses for the lens holder 5 to move to the desired position is applied to the piezoelectric element 1, or a pulse voltage is applied to the piezoelectric element 1 for a time necessary for the lens holder 5 to move to the desired position.
In the SIDM, however, the movement of the lens is performed by the friction between the lens holder 5 and the drive shaft 3 and reciprocation, raising a problem such that the moving speed of the lens varies. For example, even if the average moving speed of the lens is 3.75 mm/sec, the moving speed varies from the minimum (2.2 mm/sec) to the maximum (5.3 mm/sec) at a normal temperature. As the moving speed of the lens holder 5 on which the lens is mounted varies, the moving distance of the lens holder 5 varies even if the pulse voltage is applied to the piezoelectric element 1 by the specified number of pulses, or for the specified time. Accordingly, it is difficult to move the lens to the desired position. In particular, in macro shooting in which the moving distance of the lens holder 5 from the infinity end becomes large, a variation in moving distance becomes extremely large, making it difficult to control the moving distance by the pulse voltage.